You’re going to have to tell her that. The problem is, you’ve already set precedent and been allowing it to happen. Has she been interrupting even if the door is closed? Or is that a proposed solution?
The relationship between son in law and mother in law is one of inherent respect (or should be!) so it’s understandable you find it hard to tell her she’s been doing something all wrong. To change her habit, tell her work has ramped up the pressure and you will need to focus intently, and will need to be left alone if the door is shut. In fact, announce it to the whole family rather than singling her out. Frame it so that there’s been a change at work rather than letting her know she has been a bother all this time. It’s a forgivable white lie.
As a mother in law myself I can tell you we hate to think we are a burden. At the same time, as older women, we look to the younger male head of the family for help. Reasonable limits can be put on that and you can try to draw the boundary in a way that spares her feelings. But the priority has to be on your ability to function as breadwinner. (Or one of them; odds are you earn more.)
Make a do not disturb sign and put it on the door when it’s shut, as a reminder of the new policy due to your “increased work pressure”. After a couple weeks the “temporary” increase in work pressure should become a new routine, she’ll get the message.